The present invention relates to a heat resistant iridium-doped Ni-base superalloy. More specifically, the invention relates to a heat resistant iridium-doped Ni-base superalloy that is effective to improve output power and efficiency of a high-temperature apparatus when used as a gas turbine for power generation, a jet engine, a rocket engine and so on.
A heat resistant Ni-base superalloy is an alloy containing Ni as a basic constitutional element, to which main constitutional elements, such as Co, Cr, Mo, W, Al, Ti, Ta, Nb, Re, Hf and so on, are contained.
Since the heat resistant Ni-base superalloy has an excellent mechanical strength at high temperatures. For example, it is used as a turbine blade, a turbine vane and so on of a gas turbine for power generation, a jet engine and a rocket engine. In order to improve the output power and the efficiency of high temperature apparatus, it is the most effective to increase the operating temperature of the combustion gas, and improvement in high temperature properties of an heat resistant Ni-base superalloy is the exigent task to realize such increase.
The improvement in high temperature properties should be verified by two standpoints, i.e., the high temperature strength and the high temperature corrosion resistance.
In order to improve the high temperature strength of the heat resistant Ni-base superalloy, the addition of W, Mo, Ta, Re and so on, for example, has been attempted. However, it has been confirmed that the excessive addition of these elements promotes the precipitation of a harmful phase, because it deteriorates the microstructural stability of the alloy, and accordingly the strength of the Ni-base superalloy is lowered as contrary to the intention.
The improvement in high temperature corrosion resistance is another important problem since the material is used in a highly corrosive atmosphere. For example, turbine blade of a gas turbine are exposed in a severely oxidative gas atmosphere due to combustion. Furthermore, since a fuel contains sulfur and a thermal electric power plant is generally located near a coast, the blades are also exposed in a corrosive atmosphere due to the combustion gas including a large amount of salt.
In order to improve high temperature corrosion resistance of the blades that are used under such a severe oxidative and corrosive atmosphere, it is dangerous to depend only on a coating having good corrosion resistance unless it is guaranteed that the coating layer is not broken. To improve the high temperature corrosion resistance of the Ni-base-superalloy itself is the more reliable solution.
The objective of the invention is to provide a heat resistant Ni-base superalloy that has excellent high temperature strength and high temperature corrosion resistance.
The invention provides a heat resistant Ni-base superalloy that has excellent high temperature strength and high temperature corrosion resistance by adding iridium having a high melting point.
When iridium (Ir) is added, the alloy structure is arrayed to maintain structural stability well, and the precipitation strengthening enchanced. At the same time, iridium dissolved in the xcex3 phase and the xcex3xe2x80x2 phase to proceed solid solution strengthening. Iridium has the race-centered cubic structure, which is the same as Ni, and therefore easily substitutes for Ni. W, Mo, Ta and the like, which have been used as the alloying elements, have the body-centered cubic structure, and Re and the like have the close-packed hexagonal structure, which is considered to be one of the reasons of lowering the structural stability.
Accordingly, the iridium-added heat resistant Ni-base superalloy has an excellent high temperature strength, and can withstand the use under a high temperature and a high stress.
Furthermore, iridium has a high melting point and exhibits a small diffusion coefficient at a high temperature. Therefore, the deterioration of the characteristics of the heat resistant Ni-base superalloy is thus suppressed, and the high temperature corrosion resistance is improved.
The amount of iridium added is necessarily at least 0.1 atomic % to sufficiently exhibit the improvement in high temperature strength and high temperature corrosion resistance. On the other hand, the upper limit is not particularly strict, and can be appropriately adjusted depending on the use of the Ni-base superalloy. In general, when the amount exceeds 5 atomic %, the specific density is increased, and it affects the price. Therefore, with respect to the amount of iridium, between 0.1 atomic % and 5 atomic % can be preferably exemplified.
As the heat resistant Ni-base superalloy itself, various kinds thereof can be employed. For example, TMS-63 (6.9Cr-7.5Mo-5.8Al-8.4Ta-balance Ni (weight %)) as one or an Ni-base single crystal alloy, Mar-M247 (10Co-10W-8.5Cr-0.7Mo-5.5Al-3Ta-1.4Hf-0.16C-0.02B-0.1Zr-balance Ni (weight %)) as one of Ni-base polycrystalline alloys, and the like are exemplified.